Re: VLF: Near field boundary (Re DK7FC's DFCW600 8.97kHz transmission NOW)

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Dear Markus,

Maths, EM theory and Maxwell's equations were not my strong points at all, so I'm struggling to understand what you are saying (this is my weakness and not a weakness in your explanation).

I think you are saying that at the sort of distances being achieved by Stefan (~5.2kms) , the H-field component would be slightly weaker than the E-field, so an E-field probe would result in marginally better reception? At greater distances, the difference would not be significant.

Markus, if you have time, I have some more questions:

• We think Stefan's earth electrode signals are "making the hop" without being aided by buried utilities. Is there any way of confirming this from the field strengths measured using both H and E-field antennas?
• Is Stefan likely to get much further using his earth electrode antenna?  i.e, by radiation
  
• Do you know (from theory) how a 300m high Marconi vertical and a 300m long earth electrode antenna are likely to compare as antennas radiating a far field signal?

Thank you everyone who has been contributing to the discussion and practical experimentation below 9kHz. It is totally absorbing. I just wish I was more brainy!

73s
Roger G3XBM

On 26 June 2010 16:11, Markus Vester <markusvester@aol.com> wrote:

Dear Roger, Stefan,

actually, with all else equal, the earth antenna signal would have been 3 dB
weaker on a loop receiver! This is due to a small effect at the near field
boundary, mentioned by Jacek two weeks ago.

The field equations for a small electrical dipole are

E(r) = il *Zo / 4pi * (jk)² * [1/(jkr)³ + 1/(jkr)² + 1/(jkr)] * exp(-jkr)
H(r) = il / 4pi * (jk)² * [1/(jkr)² + 1/(jkr)] * exp(-jkr)

with E and H being the transverse fields in the equatorial plane,
il = source moment = current times effective height,
Zo = 377 ohm,
j = imaginary unit (note that j³ = -j)
r = distance from the antenna,
k = wave number = omega/c = 2pi/lambda.

The appended image shows the fields around a one meter high Marconi driven
by one ampere, thus radiating 1.42 uW (EMRP) at 9 kHz. The E field (red
line) is dominant near the antenna, but has a slight ditch at 5.3 km. Here
the electrostatic r^-3 and the radiative r^-1 components cancel, and only
the inductive r^-2  component is left. On the other hand, the H-field (blue
line, scaled to E-field units by 377 ohms) does not have a r^-3 component in
the first place, and no cancellation occurs. In the^far field, E and scaled
H are of course asymptotically equal, with magnitudes proportional to il,
frequency (jk) and r^-1.

For the dual case of a magnetic source (eg. earth antenna) of same ERP, the
roles are simply interchanged, so red would be magnetic, and blue electric
field.

The noise level indicated by the dotted line (5 dBuV/m/sqrtHz) was measured
ten years ago and is probably about 10 dB too low. In case you prefer
magnetic units (picotesla), the conversion is 1 V/m = 3.33 nT, or 0 dBuV/m
= -70.5 dBpT.

Well done, Stefan!

Kind regards,
Markus (DF6NM)


From: Roger Lapthorn
Sent: Saturday, June 26, 2010 12:52 PM
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: DK7FC's DFCW600 8.97kHz transmission NOW

Stefan,

I wonder how strong the signal would have been on a RX loop rather than the
E-field probe? This would be worth checking.

73s
Roger G3XBM


On 26 June 2010 11:50, Roger Lapthorn <rogerlapthorn@gmail.com> wrote:

73 - the first message proper on the Dreamer's Band Stefan?

Kind regards
Roger G3XBM

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